Terrain elevation maps are frequently used in aviation for purposes of maintaining pilot spatial situation awareness, and for other purposes that include airborne radar signal processing. For display, or for processing purposes, the map of terrain is represented on a two dimensional planar display in an x/y Cartesian format, even though the terrain is superimposed on the spherically curved earth surface. Because the radius of curvature is large, and the size of the region represented by the map is small with respect to the radius of the earth, the “flat earth” representation of the terrain elevation map is acceptable.
The use of terrain elevation data in a Cartesian format is complicated by the fact that terrain data is typically provided as points uniformly sampled in latitude and longitude. It is often the practice to present terrain data in the latitude/longitude format as an approximation to the desired x/y format. This representation of terrain elevation in the latitude/longitude format is reasonably good at latitudes near the equator. However, terrain elevation data represented in the lat/long format presents some processing anomalies at other latitudes.
Typically, a terrain elevation map in an aircraft, or in any moving vehicle, represents the terrain over a limited size region in the vicinity of the vehicle. The size of the region depends on display, or other, requirements. As the vehicle moves, it is necessary to update the map data to incorporate the new terrain. If the map is maintained with an orientation referenced to true north, the reference frame of a generated map has to rotate to maintain the orientation to true north as the vehicle travels. This will occur at any latitude except at the equator. As a result, if the map is maintained referenced to north, a map update that includes this reference frame rotation must consist of re-building the entire map in order to maintain the north reference. This is necessary despite the fact that new terrain entering the mapped region may constitute a relatively small portion of the map. Re-building the entire map can result in substantial computation burden and cost.
Therefore, there exists a need to provide a terrain elevation map that does not introduce excessive distortion, and which permits an incremental update to the map as the vehicle moves that prevents the need to periodically re-build the map.